This invention relates generally to solid-state lasers. More particularly, this invention pertains to the thermo-mechanical properties of the optical material YAG, an acronym for yittrium aluminum garnet (Y.sub.3 Al.sub.5 O.sub.12), the most commonly used solid-state laser material in the world.
Conventional YAG lasers, diode-pumped or flashlamp-pumped, have limits to the amount of average power they can produce. The limits are determined by the amount of heat generated during the optical pumping process, and by the strength of the material being pumped. Heat generation in such lasers is described in T. Y. Fan, "Heat Generation in Nd:YAG and Yb:YAG", IEEE Journal of Quantum Electronics, Volume 29, pages 1457-1459, 1993 and D. C. Brown, "Heat, Fluorescence, and Stimulated-Emission Power Densities and Fractions in Nd:YAG", IEEE Journal of Quantum Electronics, Volume 34, pages 560-572, 1998.
During high-average-power operation, a great deal of heat is generated. Heat is normally removed from the barrel of the rod or the surfaces of a slab. The rod or slab is conventionally cooled by flowing temperature controlled water through a surrounding channel or sheath. This causes a temperature gradient between the edge of the rod or slab which is coolest and the center which is hottest. The temperature gradient in turn produces stresses in the rod or slab that can become comparable to the intrinsic strength of the YAG. The strength of YAG is lowest on its barrel and faces that have been optically finished or ground. Rod barrels are typically ground but can also be polished. In either case defects, scratches and voids are found in the barrel that reduce the strength. Because polished surfaces have statistically smaller defects and scratches, they have higher strength than ground or roughened barrels. Reference is made to J. Marion, "Strengthened Solid-State Laser Materials," Applied Physics Letters, Volume 47, pages 694-696, 1985. Barrel roughening is often performed to eliminate or reduce parasitic oscillations in pulsed high-gain lasers. For CW lasers, all polished surfaces are usually acceptable.
When the thermally-induced stresses on the rod barrel become equal to the strength of the surface, the YAG material shatters and must be replaced. This limits the average power to input pumping below the fracture-limit. The amount can be calculated exactly and varies with the optical material composition, the size of the rod and the way in which the barrel surface has been prepared. The material thermal expansion coefficient, thermal conductivity, Poisson's ratio and Young's modulus figure in the calculations.